Umm, .intro was just an example, as I would have hoped you would realize.
The first thing you (and any beginner) must realize is that in many (if not most) cases, you cannot take code or examples as literal truth. You have to take what you are told and modify it to suit your needs. If you were to place your description.ext file in the literal path that I gave you as an example, your probably would spend hours trying to find the OFP directory because chances are that's not what your real directory is.
Therefore, I seriously doubt that you named your mission "MyMission", and as such, do not attempt to place the description.ext file in a "MyMission" directory! Put it in whatever directory your mission really resides.
Now, to answer your question, after you are done creating your mission, while still in the editor, click on the Save button. One of the fields is a drop-down menu. In the drop-down menu, choose "MP Mission". Then, click on the Save button (or is it labeled "OK"? I don't recall). This will save your mission as a multiplayer mission in your mpmission directory. So, the next time that you host an online game, the mission file will be automatically transferred to any clients that join your game.
Good luck.